The present invention relates to a system for density correction of medical imaging film hard copy, and in particular, to the conversion on film of images derived from a patient by an intermediate electronic means.
Medical electronic imaging systems employ a scanner that detects the transmission, emission, or reflection of radiation from or through a patient. Types of radiation used for this purpose include x-ray, gamma ray, sound, heat, light and radio waves. The types of scanners or imaging modalities include Computed Tomography (CT), Computed Radiography, Digital Radiography (DR), Digital Subtraction Angiography (DSA), Video Fluoroscopy, Nuclear Medicine, Ultrasound, Thermography, and Magnetic Resonance Imaging (MRI). These scanners convert the detected radiation to electronic signals and through computer processing into an image that is displayed on a monitor such as a video display.
A film hard copy is generated by use of a multiformat camera that uses a light source such as a cathode ray tube, laser, or light emitting diode to expose the electronic image information on photosensitive material (film). The exposed film is then chemically processed to yield a developed film hard copy of the patient's images. An exemplary multiformat camera is one sold under the trademark Multi-Imager 16 by Matrix Instruments, Orangeburg, N.Y. An exemplary film processor is sold under the trademark Curix 90C by AgfaGevaert N.V., Mortsel, Belgium. In the conventional multiformat camera, a light beam based upon the radiation data from the patient is impinged upon a sheet of film so that it "writes" the image thereon. The image ultimately created on the film is made up of areas that vary in lightness and darkness with the objective that the degree of lightness and darkness accurately represents the electronic image data generated by the medical imaging scanner. The degree of lightness or darkness in each film area is known as "film density", each value of film density ideally corresponding to the value of an electronic signal generated by the medical imaging scanner. After exposure in the multiformat camera, the film is processed through a chemical development and fixing bath so that a visible image is formed. It will be evident that accuracy throughout the image writing and film processing is required in order that the condition of a specific film density corresponding to a specific scanner electronic signal value is achieved. Otherwise, the density distribution of the film image will not correspond to that of the scanner electronic image and the accuracy of the medical diagnosis can be compromised. Determination of any density distribution inaccuracy must be made and compensated for as quickly as possible.
Inaccuracy in film density could be due to any one or all of the following primary factors.
(A) The light generated by the "writing" source may change due to component aging and changes in control signals due to heat and electronic drift.
(B) The chemicals used in the developing and fixing stages of the film processing can age.
(C) The temperature of the chemicals used in the developing and fixing stages of the film processing can change.
(D) Individual boxes of film can have different sensitivity due to batch variations and aging.
In the known systems, exposed and developed films are periodically checked to determine if they are accurate with respect to film density distribution corresponding to the scanner electronic image. When it is determined that there is an error, appropriate adjustments are made in one or more of the several stages of the system. In general, such monitoring of the film has been done manually through visual observation by the scanner technologist. Recently, however, attempts have been made to semi-automate monitoring by using test patterns and densitometric measurement. However, these methods still require an exposed and developed film to be manually measured which is not practical except as a periodic spot check.
It is an objective of the present invention to provide an integrated system that automatically measures the film density distribution of each medical imaging film hard copy.
It is a further objective of the present invention to provide an improved system of the aforementioned type in which film density distribution drift is automatically detected and automatically corrected so that subsequently exposed and developed films are accurate.